Key Takeaways
- A gallon of paint covers 350 to 400 square feet on smooth walls, less on textured surfaces
- Measure wall area by multiplying room perimeter by ceiling height, then subtract windows and doors
- Two coats of paint are standard for color changes; one coat is enough for the same color
- Ceilings, walls and trim each need separate calculations because they use different products
- One gallon of paint weighs roughly 10 pounds and covers about 400 square feet of smooth wall
Coverage refers to the number of square feet a gallon of paint can cover under standard conditions, typically 350 to 400 square feet per gallon.
Dry film thickness is the thickness of the paint layer after it has dried, which determines how many coats are needed for proper coverage.
Buying paint is one of the few home improvement tasks where the cost of the material is a small fraction of the labor. That means running out of paint mid-room is not just inconvenient. It means stopping, driving to the store, buying another gallon and losing momentum on a job that was supposed to take an afternoon.
Here is how to estimate accurately before you start.
Measure the wall area
The fastest way to get total wall area is to measure the perimeter of the room and multiply by the ceiling height. A room that is 12 by 14 feet with 8-foot ceilings has a perimeter of 52 feet:
52 x 8 = 416 square feet of wall area
That is the starting point. Now subtract the areas you will not paint.
Subtract windows and doors
A standard interior door is about 21 square feet. A standard window is roughly 15 square feet. Count them up, multiply by the number of each, and subtract from the total.
A room with two doors and three windows:
(2 x 21) + (3 x 15) = 87 square feet to subtract
416 - 87 = 329 square feet of paintable wall area
This step matters more than people realize. A living room with a large sliding glass door and several windows might have 40 to 60 square feet of glass and frame. Skipping the subtraction means overbuying by 10 to 15 percent.
Gallons per coat
The coverage rate depends on the paint and the surface. A gallon of flat latex paint covers about 400 square feet on smooth drywall. Textured surfaces like knockdown or orange peel reduce that to 300 to 350 square feet per gallon because the texture holds more paint in its valleys.
For the 329 square foot room at 400 square feet per gallon:
329 / 400 = 0.82 gallons per coat
You would buy 1 gallon for a single coat in that room. For two coats, you need 1.64 gallons, which rounds up to 2 gallons.
Two coats is the standard
One coat of paint almost never gives full coverage, especially when changing colors. The old color bleeds through, the sheen looks uneven and the finish looks thin. Two coats give you consistent color, uniform sheen and the durability the paint was designed to provide.
The exception is painting the same color over the same color. In that case, one coat is often sufficient if the existing paint is in good condition and not peeling.
Ceilings are a separate calculation
Ceilings are measured by their area: length times width. The same 12 by 14 foot room has a ceiling area of 168 square feet. Ceilings often use a different product, flat white ceiling paint, and a different coverage rate because overhead application is thicker.
A gallon of ceiling paint covers about 300 to 350 square feet. For that 168 square foot ceiling, you need about half a gallon for one coat.
Trim is measured differently
Baseboards, crown molding and door and window casings are measured by linear feet, not square feet. A room with 52 feet of baseboard and 20 feet of crown molding needs enough trim paint to cover 72 linear feet.
Trim paint typically covers about 100 linear feet per gallon for a 3-inch wide baseboard, less for wider profiles. One quart of trim paint is often enough for an entire room of standard baseboards.
Primers and specialty paints
New drywall, bare wood and stained surfaces each need a primer coat before the finish paint goes on. Primer is sold separately and has its own coverage rate, typically 250 to 300 square feet per gallon.
Dark colors over light surfaces sometimes need a tinted primer to achieve full coverage in two coats. Your paint store can tint primer to match your finish color, which reduces the number of finish coats needed.
The paint calculator takes your room dimensions, window and door counts, and number of coats to produce a complete material list with gallons needed.
Editorial note: This article is reviewed for accuracy and updated regularly. All calculations use standard paint industry formulas. For the latest coverage rates, consult your paint manufacturer directly.
Frequently asked questions
How many square feet does a gallon of paint cover?
A gallon of interior latex paint covers 350 to 400 square feet on smooth drywall. Textured surfaces, porous substrates and rough finishes reduce that to 250 to 350 square feet per gallon. The coverage rate is printed on the can.
How much paint do I need for a 12x12 room?
A 12 by 12 foot room with 8-foot ceilings has about 384 square feet of wall area. After subtracting a door and a window, you have roughly 348 square feet of paintable surface. At 400 square feet per gallon, one gallon covers a single coat, and two gallons cover two coats.
Should I buy extra paint for touch-ups?
Yes. Keep the leftover paint in a sealed container for touch-ups. Small nail holes, scuffs and marks are much easier to fix with saved paint than with a new can that might not match perfectly due to batch variation.
How many coats of paint should I apply?
Two coats is the standard for most interior walls and ceilings. One coat is sufficient when painting the same color over the same color in good condition. Primer coats are separate from finish coats and add one additional layer when needed.